Minimize Cooling Tower Blowdown
A HERO™ system for cooling tower makeup has the following benefits:
- Excellent resistance to membrane fouling, which reduces cleaning cost and membrane replacement.
- Higher product water quality because of improved rejection characteristics at the high operating pH.
- Reduced operating cost because organic biocides and antiscalant compounds are not required. Cooling tower water will contain zero hardness, which eliminates the need for antiscalants and improves heat exchanger efficiency.
- High recovery that not only maximizes the amount of water captured for reuse but minimizes the amount of water requiring disposal. In some cases without the HERO™ process (especially when evaporation is required), treatment and disposal of the waste stream can be more expensive than treatment of the major feed stream.
The maximum acceptable silica level is a controlling factor for cooling tower blowdown. Generally, 120 mg/L of dissolved silica is considered the highest level that can be maintained in a cooling tower system with relatively hard water. At higher concentrations, silica will precipitate in the piping or on heat exchanger surfaces.
While conventional cooling towers are limited to silica levels of 120 mg/L, the HERO™ process has demonstrated the ability to maintain over 1500 mg/L of silica in solution. The process takes advantage of the principle that silica solubility increases with increasing pH. Because of this increased solubility, and the fact that lime softening will remove over 50 percent of the silica prior to entering the cooling tower, the blowdown volume can be reduced by a factor of 20 to 30.
The major features of the process are:
- Recycled water is partially softened in a lime-soda softening clarifier. Lime and soda ash are added to precipitate calcium and magnesium hardness (as well as silica) at an operating pH of about 10. During this step, most of the organics will also be absorbed and removed with the sludge.
- Clarified effluent is then filtered in a multimedia filter.
- Following filtration, effluent is treated in a Weak Acid Cation (WAC) ion exchange system to remove remaining hardness to less than 1 mg/L as calcium carbonate. The WAC system is periodically regenerated with dilute hydrochloric acid to place the resin back in the hydrogen form. A small waste stream containing mostly calcium chloride and water is produced during the regeneration process. Backwash and rinse water are also produced during the regeneration process, but will be recycled to the clarifier to minimize the volume of waste requiring disposal.
- Softened and filtered water is then used as cooling tower makeup. As with most cooling towers, chlorination would be required to prevent biological growth. While in the cooling tower, the water is naturally degassified to remove dissolved carbon dioxide.
- Blowdown from the cooling tower is treated in a reverse osmosis system operating at 90 to 95 percent recovery. Caustic is added ahead of the RO system to raise the pH to approximately 10. High-quality product water (permeate) from the RO system will also be used as cooling tower makeup.
- The flow rate to treat through RO is significantly reduced by softening the water with the WAC system, and is made possible by increased silica solubility at high pH. HEROTM also operates at a higher flux (gallons per square feet of membrane per day) than conventional RO, which further reduces the equipment required. These factors combine to substantially reduce capital costs.
In summary, the HERO™ process can reduce the volume of cooling tower blowdown by a factor of 20 to 30 for many applications. Operating costs for power and chemicals are approximately $1 per 1000 gallons treated. The small volume of blowdown remaining (e.g., 10 gpm with the HERO™ process versus 250 gpm initially) can be disposed of by evaporation or other means.
Hydrometrics, Inc., 2727 Airport Road, Helena, MT 59601. Tel: 406-443-4150; Fax: 406-443-0760.